Field
The present disclosure relates to the design of an optical source. More specifically, the present disclosure relates to the wavelength control of an external cavity laser with reduced optical mode-hopping.
Related Art
Silicon photonics is a promising technology candidate for dense, large-bandwidth interconnects for use in next-generation processing and computing systems. Considerable progress has been made to develop a comprehensive portfolio of optical components for such advanced intra/inter-system interconnects, including laser light sources.
While a monolithic silicon light source remains elusive, silicon-assisted hybrid external cavity lasers have proven promising. A hybrid external cavity laser often includes a material section as an electrically pumped optical gain medium, and a silicon mirror as the other reflector in the external cavity, as well as an output coupler. This is illustrated in FIG. 1, which presents an example of an existing hybrid external cavity laser. While FIG. 1 illustrates a vertically coupled hybrid external cavity laser that uses a grating coupler to couple the light from a reflective semiconductor optical amplifier (RSOA) into a silicon optical waveguide in a silicon-on-insulator (SOI) technology, other hybrid external cavity lasers are edge-coupled and use tapered edge couplers to couple the light into the silicon optical waveguide. Note that some hybrid external cavity lasers use ultra-efficient tunable silicon photonic reflectors (e.g., silicon micro-rings or ring resonators) to control and tune the wavelength of the laser, and thus to create a tunable laser. Moreover, a directional coupler between the optical gain chip and the ring-resonant filter is used for laser output, and an optional phase tuner on silicon can be used to fine-tune the alignment of the lasing cavity mode to the ring-resonant-filter resonance.
The principle of a hybrid external cavity (silicon-assisted) laser is shown in FIG. 2. The optical cavity mode (which is one of the allowed standing waves in the optical cavity) that experiences the highest reflection from the tunable silicon ring resonator may dominate the natural optical mode competition in the laser and achieve stable lasing (as shown by the bold vertical line in FIG. 2). A current-adjustment mechanism in the hybrid external cavity laser may be used to control the current injected into the reflective semiconductor optical amplifier. This current can control the amount of output power available from the hybrid external cavity laser. Moreover, a resonance-adjustment mechanism in the hybrid external cavity laser may change the resonance wavelength of the tunable silicon ring resonator, which controls the cavity lasing wavelength and the emission wavelength.
However, tunable silicon photonic reflectors are often very sensitive to temperature changes. For example, as the current is increased, heating of the hybrid external cavity laser may occur, which can change the effective length of the external hybrid optical cavity because of the thermo-optic effect and, thus, can red-shift the cavity modes (as shown by the other vertical lines in FIG. 2). In particular, because the optical gain and laser section is long (typically, greater than 500 μm in length), the cavity modes are spaced narrowly, with multiple cavity modes present within a resonance of a single ring resonator. Although the injected current can reduce the effective index of refraction (which may partially compensate for this effect) and a tunable silicon photonic reflector may also be used to change the lasing wavelength, the overall effect of temperature is that the specific cavity mode may experience reduced reflectivity as it shifts to a longer wavelength. When another cavity mode achieves the highest effective optical gain, then this other cavity mode typically becomes the dominant lasing mode (and may lase preferentially over the previous incumbent lasing optical mode), and the hybrid external cavity laser experiences a hop in the lasing wavelength corresponding to the new cavity mode (and, in particular, a sudden laser optical-mode jump to another cavity mode).
During data transmission, wavelength optical mode-hops are essentially ‘glitches’ that can corrupt data transmitted, or may send data to the incorrect destination in a wavelength-division-multiplexed (WDM) system. Consequently, optical mode-hopping is undesirable for digital communication links and typically needs to be reduced before a tunable hybrid external cavity laser may be effectively used for transmitting high-speed data across a range of wavelengths with good fidelity across the communication channel.
One approach for maintaining the alignment between the ring resonator and a lasing cavity mode is to minimize the output at monitoring port(s). For example, a closed-loop controller can maintain the alignment by using the monitor-port output power as a feedback signal to control the ring-resonator heater or the cavity phase tuner in order to minimize the optical power at the monitor port(s). While this approach is simple, efficient and has low power consumption, the feedback is relative because of the symmetric shape of the monitor output or error signal. Consequently, there is no single absolute value indicating perfect alignment. This means that the feedback signal has no ‘direction’ information that can be used to guide the controller to correct any errors that occur. Therefore, a so-called ‘try and check’ type of control technique (e.g., a so-called ‘bang-bang’ feedback technique) may be needed for closed-loop control.
Furthermore, the controller usually relies on a prior feedback signal (such as a previous sample, T−1) as a reference to determine the appropriate correction for the current sample (T). However, laser-power change due to other effects (such as laser-current change) can result in the wrong corrective action and, thus, the usefulness of this power-monitoring approach may be limited to situations where the optical power is not fluctuating.
Hence, what is needed is a hybrid external cavity laser without the above-described problems.